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The vulnerability resides in Symfony’s NoPrivateNetworkHttpClient, which is intended to block HTTP requests to private IP addresses. It relies on `IpUtils::PRIVATE_SUBNETS` to identify these addresses. However, this list fails to include several IPv6 transition mechanisms—6to4 (2002::/16), Teredo (2001::/32), NAT64 (64:ff9b::/96 and 64:ff9b:1::/48), and IPv4-compatible IPv6 (::/96). These mechanisms can embed private IPv4 addresses (e.g., 127.0.0.1) within an IPv6 address. Because `IpUtils::checkIp6()` performs only a bitwise comparison against the constant list and does not extract the embedded IPv4 address, it incorrectly treats these addresses as public. This allows an attacker to supply a URL like http://[2002:7f00:1::]/` (which decodes to 127.0.0.1 via 6to4) and bypass the security boundary, leading to Server-Side Request Forgery (SSRF). The actual reachability of the IPv4 address depends on the deployment's IPv6 routing, but the security boundary is crossed regardless. The fix involves adding these missing IPv6 transition prefixes to the blocklist. Patches are available for branches 5.4, 6.4, 7.4, 8.0, and 8.1.
<h2 style="color: blue;">DailyCVE Form:</h2>
Platform: Symfony Framework
Version: 5.4.0-5.4.52
Vulnerability: SSRF Bypass
Severity: Critical
date: 2026-05-27
<h2 style="color: blue;">Prediction: 2026-05-27</h2>
<h2 style="color: blue;">What Undercode Say:</h2>
Test for IPv6 transition form bypass using 6to4 encoding (127.0.0.1) curl http://[2002:7f00:1::]/ Test using NAT64 encoding (127.0.0.1) curl http://[64:ff9b::7f00:1]/ Test using IPv4-compatible encoding (127.0.0.1) curl http://[::7f00:1]/ Test using Teredo encoding curl http://[2001::1]/
The `NoPrivateNetworkHttpClient` will allow these requests because `IpUtils::isPrivateIp()` returnsfalse, despite them targeting 127.0.0.1.NoPrivateNetworkHttpClient
<h2 style="color: blue;">Exploit:</h2>
An attacker can exploit this vulnerability by controlling a URL that is passed to. By using any of the IPv6 transition forms (6to4, Teredo, NAT64, or IPv4-compatible) to encode a private IPv4 address (e.g., 127.0.0.1, 10.0.0.1, 192.168.1.1), they can cause the Symfony application to make an HTTP request to that internal address. The application will treat the request as if it were going to a public IPv6 address, bypassing all restrictions. This can be used to scan internal networks, access internal services, or perform other malicious actions that rely on internal network access.2002::/16
<h2 style="color: blue;">Protection:</h2>
Update Symfony to a patched version. The following versions contain the fix:
- 5.4.53 and higher
- 6.4.41 and higher
- 7.4.13 and higher
- 8.0.13 and higher
If an immediate update is not possible, implement a custom URL validation mechanism that explicitly rejects any IPv6 addresses belonging to the,2001::/32,64:ff9b::/96,64:ff9b:1::/48`, and `::/96` prefixes. Additionally, ensure your network infrastructure (routers, firewalls, etc.) is configured to block or drop traffic destined for these IPv6 transition prefixes.
Impact:
Successful exploitation allows an attacker to perform Server-Side Request Forgery (SSRF) attacks. This can lead to:
– Internal Network Scanning: The attacker can probe internal network ports and services to discover vulnerabilities.
– Access to Internal Services: The attacker can interact with internal APIs, databases, or other services that are not exposed to the public internet.
– Data Exfiltration: Sensitive data that resides on internal systems may be exposed.
– Potential for Further Exploitation: SSRF is often a stepping stone for more severe attacks, such as Remote Code Execution (RCE) or privilege escalation.
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Sources:
Reported By: github.com
Extra Source Hub:
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